This invention relates to ambient temperature, rechargeable, non-aqueous, all-inorganic electrochemical cells. More specifically, this invention relates to such cells utilizing a new type of electrode in which the active material consists entirely of one or more nonmetallic compounds or salts of the electropositive species on which the cell is based (e.g., Li.sup.+, Na.sup.+), which is typically the same as that of the main charge-carrying species in the electrolyte. In addition, this invention relates to ambient temperature, non-aqueous, inorganic electrolytes for use in cells based on this new electrode type.
The ultimate goal of the research underlying the present invention is to develop improved rechargeable batteries operating at or near room temperature that provide high specific energy and power densities suitable for electric vehicles. To allow for a wide range of ambient conditions, the desired temperature range for electric vehicle batteries as envisioned in the long term by the U.S. Advanced Battery Consortium (USABC) is -40 to 85.degree. C. At present, the lead-acid battery is the leading candidate for full-scale on-the-road electric vehicles due to its mature yet continually evolving technology and well-established manufacturing base. Its chief limitation, however, is a low specific energy which stems from a low cell voltage due to its use of aqueous electrolytes and the relatively high cell component material molecular or formula weights. Thus, worldwide efforts have been in progress to develop alternate battery chemistries that provide higher specific energy and power densities as required to insure the long term economic viability of electric vehicles.
Lithium is among the most promising of rechargeable battery electrode active materials because of its high standard potential and low electrochemical equivalent weight. For many years, ambient temperature rechargeable lithium batteries have been in an ongoing state of research and development to provide lightweight, economical power sources for a variety of applications ranging from notebook computers and heart pacemakers to full-scale aerospace and transportation needs. A recent review of all the different approaches taken to date in the design of ambient temperature rechargeable lithium batteries is provided by Hossain (Chap. 36 in Handbook of Batteries, 2nd ed., ed. by D. Linden, McGraw-Hill, Inc., 1995).
From a review of the patent literature and other published studies pertaining to advanced batteries considered for use in electric vehicles, it appears that the majority of research in ambient temperature lithium rechargeable batteries has been concentrated almost exclusively on two main types of cells which differ according to the form the lithium active material assumes during cell operation, i.e., i) those using lithium metal anodes, or ii) those using certain solid materials for both electrodes that can reversibly intercalate Li.sup.+ cations. Both types of cells may utilize a variety of liquid or solid (e.g., polymer) electrolytes. In type (ii) cells, often referred to as Li-ion ("lion") cells, Li.sup.+ cations are shuttled back and forth between the electrodes during charging and discharging, and no free lithium metal is present. Li-ion cells often utilize porous carbon at the anode and lithiated first row transition metal oxides (e.g., Li.sub.x MnO.sub.2) at the cathode, but many deviations from this basic design exist, e.g., certain lithiated transition metal compounds with potentials sufficiently close to that of metallic lithium (e.g., Li.sub.x WO.sub.2) may be used as anodes, or porous carbon electrodes may be used at both the cathode and anode, each differing in the amount of surface area. Much research has and continues to be devoted to the development of new (and/or to the improvement of existing) materials with enhanced Li.sup.+ ion intercalation storage capabilities. At present, however, neither lithium metal anode nor Li-ion cells are sufficiently developed for large-scale commercial use in electric vehicle batteries. For lithium metal anode batteries, safety problems associated with metal dendrites abound, and for Li-ion-type batteries, current limitations regarding long-term storage and specific energy and power density need to be overcome.
The present invention, which makes use of all-metal salt electrodes, is a significant departure from conventional battery designs. A review of the prior art shows that there are relatively few designs using lithium and other lightweight, electropositive metals in which the electrode active metal assumes the form of a distinct salt phase during some stage of cell operation. U.S. Pat. No. 4,154,902 by Schwartz describes both primary and rechargeable ambient temperature, non-aqueous cells in which, during the charging stage, the electrode active material is in the form of a dithionite salt of an alkali or alkaline earth metal. In the cell design of Schwartz, the dithionite salt (e.g., Li.sub.2 S.sub.2 O.sub.4) is dissolved in a suitable anhydrous solvent together with another salt of the same metal with a higher solubility (e.g., LiClO.sub.4) to enhance the electrolyte metal cation conductivity, and SO.sub.2 is usually added at saturation. During charging, the electrode active metal is deposited in metallic form at the anode and SO.sub.2 is produced at the cathode. During discharging, the dithionite salt is reformed from metal cations produced at the anode by oxidation of the metal and S.sub.2 O.sub.4.sup.2- anions produced at the cathode upon reduction of SO.sub.2. Throughout cell operation, a steady supply of dithionite salt is provided by the battery design which employs a system for forced circulation of the electrolyte.
It is well known that in primary lithium metal anode cells employing SO.sub.2 as the cathode active material, lithium dithionite salt, which has a low solubility in SO.sub.2 as well as in most other electrolyte solvents, is typically formed during cell discharge and is deposited as an electronically insulating layer (but with some ionic conductivity) on the cathode current collector or substrate. Cell failure in such systems often occurs when the cathode current collector is entirely or almost entirely covered with solid Li.sub.2 S.sub.2 O.sub.4. Under these conditions, further cell operation is not possible, and cells in which the cathode current collector is coated with solid Li.sub.2 S.sub.2 O.sub.4 are generally not considered to be rechargeable.
Schwartz's invention is of interest in that it teaches that it is possible to utilize the reaction product of spent anode active metal and cathode depolarizer (i.e., dithionite salt) as an electrode active material in rechargeable cells, at least in some systems. However, Schwartz's invention differs fundamentally from the present invention in that the anode active metal is not always present in oxidized form but rather undergoes repeated oxidation and reduction during cell cycling (as in all metal anode cell designs). Also, the electrode active metal dithionite salt appears to be utilizable in Schwartz's cells only in the form of an electrolyte solute, and no dithionite (or other) salt phase is deposited in solid form at either electrode at any stage of cell operation.
Another important aspect in which the present invention differs from that of Schwartz is that, in the latter invention, the rechargeable version of Schwartz's cell is limited to only SO.sub.2 since the cell chemistry is based on the use of dithionite salt as active material, whereas in the present invention, a wide variety of liquid cathode materials can be utilized, and in combination with a wide variety of electrode solid phase compositions which are generally not restricted or fixed by the chemical composition of the liquid cathode employed. As discussed in more detail below, it is far more difficult, both in theory and in practice, to utilize an oxychloride as the cathode active material in rechargeable cells than it is to use SO.sub.2. Hence, in that connection, it should be noted that it is only in the primary versions of Schwartz's cell design (wherein temporary use is also made of dithionite, but only as a precursor to the formation of lithium metal anodes in situ) that SOCl.sub.2 and other oxychlorides can be used as cathode depolarizers; such substances are not an integral part of any of Schwartz's rechargeable cells.
U.S. Pat. No. 4,520,083 by Prater et al. describes an ambient temperature, non-aqueous, rechargeable cell having a reactive metal anode of the second kind, i.e., one that forms an insoluble product upon discharge in combination with a suitable electrolyte. In the cell design described therein, M.sup.+ cations (where M is the electrode active metal, e.g., Li) are expelled from the metal anode during discharging, and they immediately react with X.sup.- anions which are present in the electrolyte at a concentration much greater than that of M.sup.+ cations to form an insoluble metal salt, MX, which precipitates or deposits back on the anode. The electrolyte of this invention generally also contains a cathode depolarizer such as SO.sub.2. At the positive electrode, the cathode depolarizer is reduced to a product which can be either soluble or insoluble in the electrolyte. During charging, the slightly soluble active metal salt on the anode (MX) redissolves, the X.sup.- anions return to the electrolyte solution, and the active metal cation M.sup.+ is reduced back to metal form at the anode. At the positive electrode, the reduction product is reoxidized back to the original cathode depolarizer. As discussed by Prater et al., a low solubility in the electrolyte of both M.sup.+ cations and MX salt is believed to be necessary for this type of cell to work as effectively as possible. This condition is met by preparing electrolyte solutions which are at or near saturation in both M.sup.+ and MX. A large concentration of X.sup.- relative to M.sup.+ is provided for by adding another supporting salt, RX (wherein R is different from M), which is substantially more soluble in the electrolyte than MX. To bring M.sup.+ to saturation, a salt of M may be added to the electrolyte.
The cells of Prater et al. make use primarily of lithium (as well as other alkali and also alkaline earth metals) as the anode active material and multi-component electrolyte solvents consisting primarily of inorganic or organic compounds consisting of Group IIIA, IVA, VA, and VI elements, e.g., nitriles, ethers, cyclic ethers, sulfur oxides, and sulfur oxyhalides. For the cathode depolarizer, a number of possible redox couples were mentioned in the disclosure, e.g., Ag.sup.+ /Ag, X.sub.2 /X.sup.- (where X is a halogen), Fe(CN).sub.6.sup.3- /Fe(CN).sub.6.sup.4-, and thianthrene cation/thianthrene, but the Examples employ only SO.sub.2 as a cathode depolarizer which was cited as being the most preferred.
The invention of Prater et al. differs fundamentally from the present invention in that the anode active metal species, M, undergoes repeated oxidation and reduction during discharging and charging, as in conventional cells; hence, the anode active metal is not in the form of a solid salt phase at both electrodes during any stage of cell operation. Also, the present invention and that of Prater et al. differ in the type of predominant ionic charge carrier in the electrolyte; in the latter invention, M.sup.+ cations are not transported back and forth between the electrodes. Finally, it appears that Prater's invention is restricted to simple and demonstrably reversible redox couples such as, e.g., X.sub.n /X.sup.m- or X/X.sub.n.sup.m- (where m and n are integers), in contrast to the present invention.
It is well known that not only sulfur dioxide (SO.sub.2) but also thionyl chloride (SOCl.sub.2) and sulfuryl chloride (SO.sub.2 Cl.sub.2), have long been used as liquid cathodes in lithium primary batteries designed for very high specific energy and power density applications. In the development of improved electric vehicle batteries that can meet future energy and power requirements, it would be highly desirable to be able to exploit the promising properties of such cathode active materials in ambient temperature, rechargeable cells. Of these three liquid cathodes, the oxychlorides, i.e., SOCl.sub.2 and SO.sub.2 Cl.sub.2, are more preferred since they provide the highest cell voltages, but up until now, their use has been limited almost exclusively to primary cells. Sulfur dioxide (SO.sub.2) is the only one of these liquid cathodes that has been utilized to any significant degree in lithium and other anode active metal rechargeable batteries; this is most likely due to the much higher degree of reversibility of its redox couple. Besides those designs described above, rechargeable cells using a lithium metal anode, a catholyte of SO.sub.2 containing a salt such as LiAlCl.sub.4 added to impart a high Li.sup.+ conductivity, and carbon as cathode current collector are also known, e.g., the system described in U.S. Pat. No. 4,513.067 by Kuo et al.
U.S. Pat. No. 5,260,148 by Idota describes and ambient temperature rechargeable battery in which the anode active material consists of one or more lithium compounds or salts, Li.sub.p X, which are substantially insoluble in the electrolyte based on organic solvent. Here, X is an anion which may be either singly atomic or polyatomic and p is the anion valence of X. The electrolyte consists of an organic solvent containing a compound, A.sub.q Y.sub.r, where A is a cation which may be either singly atomic or polyatomic, and Y is an anion which may be either identical with or different from X provided that the lithium salt of Y is substantially insoluble in the electrolyte. The cathode active material consists of an anion-doped compound or compound containing a cation which is the same as A of the compound A.sub.q Y.sub.r in the electrolyte. In the Examples provided, A is a polyatomic organic cation such as tetrabutylammonium, tetraethylammonium, tetrapentylammonium, tetrabutylphosphonium, N-methylpyridinium, and N-methylpicolinium
When A is Li, there are two possibilities for an inorganic Li-containing cathode material according to Idota's disclosure, i.e., it may consist of a Li-doped transition metal chalcogenide such as Li.sub.x Mn.sub.2 O.sub.4 or Li.sub.x CoO.sub.2, or "a cathode material mixture may be prepared from the cathode active material by mixing it with the same ingredients as those used for preparing the anode material mixture." For A=Li, however, this implies that the electrolyte conductivity cannot be made very high, since the main claim specifies that the lithium salt of Y must have a low solubility in the electrolyte. Specific examples of cells in which A is Li were not provided, but in the disclosure, Idota states that for such cells, the electrolyte "is preferably based on a combination of the lithium compound and an inorganic lithium solid electrolyte. Further, lithium may be used therewith in such an amount that it dissolves slightly."
Idota's invention differs fundamentally from the present invention in several key aspects. Idota's invention for anode active material is restricted to organic solvent electrolytes. As will be shown, the present invention provides a wide variety of recipes for the electrode salt mixtures, far more than envisioned by Idota. Whereas Idota's invention is restricted to the use of Li as the anode active metal species, the present invention has been demonstrated on a wide variety of anode active metal species. Also, various additives such as, e.g., aliovalent salts that give rise to substantially improved electrode properties were discovered and developed, and are disclosed herein. The optimization of multi-component all-metal salt electrode compositions for A=Li is not considered in Idota's invention.
The present invention also makes advantageous use of certain liquid cathode materials which participate in a key way in the electrode half cell reactions, whereas Idota's invention is restricted to organic solvent electrolytes. These liquid cathodes include those for which the half cell reactions are highly irreversible, i.e., the sulfur oxychlorides SOCl.sub.2 and SO.sub.2 Cl.sub.2, and hence not amenable to conventional rechargeable cell designs. Such liquid cathodes would also be utilizable in Idota's invention for A=Li with either of the two possible types of inorganic cathode materials mentioned above, but that possibility was not mentioned in Idota's disclosure.
The present invention also addresses the problem of electrode-electrolyte compatibility, whereby for a given electrode material, suitable electrolytes are used so that high exchange current densities at the electrode-electrolyte interface can be realized. The exchange current density provides a measure of how quickly the determining half-cell reactions can take place, and thus, high exchange currents are well known to be necessary for achieving high current carrying capabilites and high power capacities in electrochemical cells. The present invention utilizes a variety of inorganic electrolytes which give rise to highly promising cell properties believed to arise from high exchange current densities. As will be shown, these electrolytes include not only the sulfur oxychlorides, for which such high exchange currents have previously been manifested by the high performance capabilities of Li primary batteries, but also certain newly-discovered ambient temperature molten salt electrolytes disclosed herein. Thus, another key aspect of the present invention is the discovery and development of compatible electrolytes for the metal salt electrodes suitable for larger size, high energy and power density rechargeable cells, a problem which Idota's invention does not address.
In contrast to SO.sub.2, the oxyhalide solvents, particularly SOCl.sub.2, have rarely been used in rechargeable batteries. This is because their practical use in such systems has generally been precluded by the high degree of irreversibility of the electroreduction reaction, which makes the in situ regeneration of any spent liquid cathode solvent impracticable. For primary lithium metal anode-SOCl.sub.2 cathode cells, the overall electroreduction reaction is EQU 4Li+2SOCl.sub.2.fwdarw.4LiCl+S+SO.sub.2 (1)
where the LiCl is deposited at the positive electrode current collector (typically porous carbon). A prerequisite for the use of either SO.sub.2 Cl.sub.2 or SOCl.sub.2 as a liquid cathode in any rechargeable cell is that some mechanism be provided for the facile regeneration of spent cathode from the electroreduction products. For SOCl.sub.2, a mechanism would have to be provided for the recombination and re-reaction of three distinct chemical species, i.e., S, SO.sub.2, and Cl.sub.2. For SO.sub.2 Cl.sub.2, the only electroreduction products are SO.sub.2 and Cl.sub.2, and hence, the reformation of SO.sub.2 Cl.sub.2 is a more straightforward process than that of SOCl.sub.2 because only two species are involved. Also, the SO.sub.2 Cl.sub.2 reformation reaction is known to be catalyzable by carbon.
Even for SO.sub.2 Cl.sub.2, however, very few studies have been published on the use of such liquid cathodes in rechargeable cells. In one study by by Smith et al. (J. Electrochem. Soc., 137, 602 [1990]), the rechargeability of the Li--SO.sub.2 Cl.sub.2 couple was demonstrated at room temperature on small prototype cells with lithium metal anodes and carbon cathodes and test cells with all-lithium reference, test, and counter electrodes using 1.5 M LiAlCl.sub.4 --SO.sub.2 Cl.sub.2 as the catholyte. Both types of cells were found to exhibit moderately good rechargeability and efficiency, but in all cases, the number of cycles was generally limited to well below 60 due to cell failure. As discussed by Smith et al. and shown by the data, there appear to be two main causes of failure in Li--SO.sub.2 Cl.sub.2 cells, i.e., lithium metal dendrite formation, and a slow rate of regeneration of spent SO.sub.2 Cl.sub.2 from Cl.sub.2 and SO.sub.2. The latter phenomenon may occur in conjunction with certain deleterious side reactions such as Cl.sub.2 attack of Li, resulting in a gradual depletion of SO.sub.2 Cl.sub.2 from the cell. Whichever mode of failure predominates appears to be dependent on a number of interrelated factors, including the prior state of cell charge, the inter-electrode separation distance, and whether the cell is anode- or cathode-limited.
In the Smith et al. study, the effects of changes in catholyte composition were studied for the small prototype cells. Increasing the LiAlCl.sub.4 concentration from 1.5 to 3.0 M was found to double the cathode cycle life, which was attributed to a faster dissolution of lithium dendrites in the more corrosive 3.0 M electrolyte. Adding SO.sub.2 was found to have no effect on the cathode efficiency, but the discharge voltage regulation was greatly improved. This improvement was attributed to the suppression of SO.sub.2 Cl.sub.2 dissociation by SO.sub.2 which eliminates dissolved Cl.sub.2 from the catholyte, thus restricting the cathode reduction reaction to SO.sub.2 Cl.sub.2 alone rather than a mixture of Cl.sub.2 and SO.sub.2 Cl.sub.2. Adding Cl.sub.2 and/or SOCl.sub.2 to this system failed to regulate the discharge voltage and degraded cathode cycling efficiency to half the baseline electrolyte value. This undesirable effect might be due to the type of anodes used in the cells of the Smith et al. study, i.e., metallic lithium, which may act more as chlorine scavengers rather than as promoters of the in situ regeneration of the SO.sub.2 Cl.sub.2 cathode.
U.S. Pat. No. 4,894,298 by Vukson et al. describes a high temperature alkali metal plus halide rechargeable cell with SO.sub.2 Cl.sub.2 catholyte. The most typical version of this cell consists of a negative electrode of sodium metal and a positive electrode which includes a solid NaCl and SO.sub.2 Cl.sub.2 catholyte to which NaAlCl.sub.4 and AlCl.sub.3 are usually added to impart a high Na.sup.+ ionic conductivity. During charging, Na.sup.+ cations are released from the NaCl at the positive electrode and are reduced at the negative electrode to metallic form. Also, a volatile reaction product is produced which is believed to consist mostly of Cl.sub.2 gas formed at the positive electrode by anodic oxidation of the Cl.sup.- anions from the NaCl. During discharging, the sodium metal at the negative electrode is oxidized to Na.sup.+ cations which migrate to the positive electrode wherein SO.sub.2 Cl.sub.2 is reduced to Cl.sup.- anions and SO.sub.2, the latter of which is believed to constitute most of the volatile reaction product that is a part of the discharging reaction. The Cl.sup.- anions combine with Na.sup.+ cations to reform NaCl at the positive electrode.
As Vukson et al. also teach, the ability to regenerate spent SO.sub.2 Cl.sub.2 catholyte in situ is essential to the rechargeability of any cell incorporating such material. Vukson et al. provide a physical means in the cell design whereby gaseous SO.sub.2 and Cl.sub.2 generated at the positive electrode during charging and discharging, respectively, can be stored and later recombined as necessary to regenerate the SO.sub.2 Cl.sub.2 catholyte. In that way, rechargeability of a cell utilizing SO.sub.2 Cl.sub.2 as a catholyte is made possible even though the types of electrode active materials employed in designs of the type utilized by Vukson et al. make it infeasible to generate SO.sub.2 and Cl.sub.2 simultaneously during cell operation. The invention of Vukson et al. is illustrative of the types of elaborate means that sometimes have to be employed to make it feasible for SO.sub.2 and Cl.sub.2 to recombine to reform SO.sub.2 Cl.sub.2.
As will be shown, an important aspect of the present invention is that it is now possible to utilize fully such high performance liquid cathodes as SOCl.sub.2 and SO.sub.2 Cl.sub.2 in ambient temperature rechargeable batteries. This aspect stems from the discovery that certain all-metal salt electrode compositions were found to work quite well with both SOCl.sub.2 and SO.sub.2 Cl.sub.2. Possible theories and chemical mechanisms underlying this discovery are given below (see "Summary of Invention"). The present invention is the first known wherein cathode redox couples more complex that those of type X.sub.n /X.sup.m- or X/X.sub.n.sup.m- can be utilized in rechargeable cells.
Besides those electrolytes based on SOCl.sub.2 and SO.sub.2 Cl.sub.2, the author also discovered several new families of ambient temperature, all-inorganic molten salt electrolytes that work well with many of the all-metal salt electrode compositions which are disclosed herein. These molten salts are based on the low-melting binary, AlCl.sub.3 --PCl.sub.5, to which either PCl.sub.3, POCl.sub.3, or PSCl.sub.3 may be added. The synergistic effects of combining all-metal salt electrodes with these electrolytes are believed to stem from higher exchange current densities at the electrode-electrolyte interface. Also, depending on the metal salts comprising the electrode solid phases, these molten salt electrolytes may also serves as liquid electrodes (usually cathodes) since they all contain components that may undergo a variety of redox reactions involving either the uptake or liberation of Cl.sup.- ions from the phosphorus-containing components (i.e., PCl.sub.5, PCl.sub.3, POCl.sub.3, and/or PSCl.sub.3). Such redox reactions are also considerably more complex than those of type X.sub.n /X.sup.m- or X/X.sub.n.sup.m-.
It is the principle object of the present invention to provide an alternative approach to the development of high specific energy and power density rechargeable batteries that are potentially suitable for electric vehicle applications. This objective is accomplished by the use of a new electrode design together with supporting all-inorganic cell component materials, and a demonstration of the viability of said electrode design and cell component materials in larger-size prototype cells.
It is a further object to provide a wide range of rechargeable cell component materials for the construction of batteries based on the new electrode design. These materials include not only new electrode compositions utilizing metal salts as the principle electrode active material, but also new ambient temperature molten salt electrolytes that possess all the desired materials properties and appear to work synergistically with most of the electrode compositions developed herein.
It is a further object to provide means, utilizing the new electrode design described herein, for the use of promising and complex liquid or soluble electrodes in a rechargeable cell. In particular, it is a further object to provide means for the exploitation of both SOCl.sub.2 and SO.sub.2 Cl.sub.2 as liquid cathodes for high specific energy and power density cells. This is accomplished by the development of compatible all-metal salt electrodes of suitable chemical composition, as well as the discovery of an electrochemical "pre-treatment" of the catholyte that further improves the cell performance.
It is a further object to provide fabrication procedures for the construction of larger-size cell prototypes that effectively utilize the most promising cell materials discovered to date.
Other objects and advantages of the invention will become apparent from the following description and examples thereof.